1. Field of the Invention:
The present invention relates to microwave circuits. More specifically, the present invention relates to surfaces used to selectively pass microwave signals.
While the invention is described herein with reference to a particular embodiment for an illustrative application, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teaching provided herein will recognize additional modifications, applications and embodiments within the scope thereof.
2. Description of the Related Art:
Some dual mode or multiple frequency band reflector antennas make use of frequency selective surfaces to direct microwave radiation from two or more feeds to the reflector of the antenna. The frequency selective surface is mounted generally parallel with the reflector between one feed with the second feed mounted between the surface and the reflector. In a transmit mode, microwave radiation from the first feed of a first frequency passes through the surface while radiation from the second feed of a second frequency is reflected by the surface to the reflector. The direction is reversed in the receive mode.
As is known in the art, frequency selective surfaces generally consist of arrays of conductive elements such as squares, circles, Jerusalem crosses, concentric rings or double squares supported by a dielectric substrate. Frequency selective surfaces are known to have several limitations. The passband of typical frequency selective surfaces is generally narrow. In addition, the conventional designs typically have slow rise and fall passband transitions.
The publication entitled "Equivalent-circuit models for frequency-selective surfaces at oblique angles of incidence"; by C. K. Lee and R. J. Langley; IEE PROCEEDINGS, Vol. 132, Pt. H, No. 6; October 1985; pp. 395-398 discloses a frequency selective surface consisting of a dielectric substrate containing an array of gridded-square printed circuit elements. The gridded-square array provides a frequency selective surface with sharp rise and fall passband transitions. However, the gridded-square frequency selective surface of Lee et al was apparently devised for separating two closely spaced and narrow frequency bands and accordingly does not appear to offer a wide passband.
There is therefore a need in the art for a wideband frequency selective surface suitable for spacecraft systems and other applications.